Efficient and even combustion is a key factor in minimizing the environmental impact of waste-to-energy incinerators, reducing both the amount of unburned material in the ash produced and the amount of air emissions. This reduction depends largely on the design of the furnace and the operating practices.
The following general guidelines foster good combustion (Licata 1986):
The grate should be covered with fuel (a uniform depth of garbage and trash) across its width. The depth at any location on the grate should be consistent with the air that can be delivered for combustion at that point. The incinerator must include an air distribution system that apportions air according to the burning rate of waste along the entire length and width of the grate. Underfire air should be introduced carefully. Depending on the technology, it can be concentrated in a small area or spread over a large area. Zones of high-pressure air and blowtorch effects should be eliminated. Bursts of air in one section of the fuel bed prevent even mixing of air in the burning refuse in other areas. Air must be introduced into burning refuse both above and below the burning bed. Oxygen provided through the overfire system helps complete the combustion of any hydrocarbons (and particulates) not oxidized near the fuel bed.
Steps must be taken to prevent the buildup of slag within the furnace. Slag can damage the boiler system and also results in poor combustion by preventing proper air mixing in the fuel bed. Gases generated in the incineration process should experience maximum mixing to facilitate oxygen reaching any unburned particles and to provide a maximum dwelling time for the gases before being released into the atmosphere.
The flue gas temperature should be at or above 1600°F for approximately 1 sec after leaving the fire bed. Figure 10.9.3 shows that these combustion conditions destroy
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